Bacterial infection in the immunocompromised patient is currently a serious problem in the United States. A complete understanding of the immunologic defense mechanisms against these "opportunistic" pathogens is essential in order to prevent or control infections in these immunologically compromised individuals. The principles of cell mediated and humoral immunity against these infectious agents have been established, and the general interactions between lymphocyte populations and the host's phagocytic cells (particularly macrophages) have been studied. However, the precise mechanisms of lymphocyte-macrophage-pathogen interactions that results in successful host defense is not adequately understood. Nocardia asteroides (an important opportunistic pathogen) provides an excellent tool for dissecting further the lymphocyte-macrophage-pathogen interactions. Purified and characterized subpopulations of lymphocytes will be fractionated from normal and immunized mice using current immunologic methodologies. Separate populations of macrophages, Kupffer cells, splenic macrophages, alveolar macrophages, and peritoneal macrophages will be collected from different organ systems. Reconstitution experiments that combine either lymphocyte subpopulations, clones, hybrids, or lymphocyte products produced by these purified lymphocytes with either Nocardia alone, macrophages alone, or macrophages plus Nocardia will be used to dissect, in vitro, the specific immunological defense mechanisms. The interactions will be analyzed by combining in vitro killing assays with studies examing parameters such as generation of superoxide anion, degree of phagosome-lysosome fusion, phagosome-lysosome pH, and levels of intracellular iron and the lysosomal enzymes, acid phosphatase and lysozyme utilizing a computerized cytospectrophotometera, and transmission electron microscopic techniques. The antigenic specificities of these interactions will be determined by using both unrelated pathogens such as Listeria monocytogenes and related pathogens such as Nocardia caviae and Nocardia brasiliensis. These experiments should provide unique and important insights into the mechanisms of lymphocyte-macrophage interactions responsible for host resistance to opportunistic pathogens.